1. Field of the Invention
This invention relates to an organic EL device which is useful for display applications, and to a method for manufacturing such a device. More specifically, this invention relates to an organic EL device which prevents intrusion of moisture from the outside environment, and exhibits excellent light emission efficiency over a long period of time, and to a method for manufacturing such a device.
2. Description of the Related Art
In recent years energetic research has been performed on organic EL devices employing light-emissive organic EL elements. Organic EL devices are expected to achieve high emission luminance and emission efficiency. This is because high current densities are achieved at low voltages. In particular, commercialization of multicolor light-emitting organic EL devices capable of high-definition multicolor display, and in particular full-color display, is anticipated in the field of display technology.
Important problems for the commercialization of organic EL devices as color displays include, in addition to the realization of high definition, device long-term stability, including color reproducibility. However, multicolor light-emitting organic EL devices have the drawback of a prominent decline in light emission characteristics (current-luminance characteristics) due to driving over a certain period.
A representative cause of this decline in light emission characteristics is the growth of dark spots. A “dark spot” is a light emission defect point. A dark spot is thought to occur due to progressive oxidation or aggregation of the material in a component layer of an organic EL element during driving or storage, due to oxygen or water within the element. Dark spot growth proceeds during current conduction, of course, but also during storage. In particular, it is thought that dark spot growth is (1) accelerated by oxygen or water existing in the outside environment surrounding the element; (2) influenced by oxygen or water existing as an adsorbent in a component layer; and (3) influenced by water adsorbed on a component used in device manufacture, or by intrusion of water during manufacture. If this growth continues, the dark spot expands over the entire light-emitting face of the organic EL device.
Conventionally, as means to prevent the intrusion of water into a component layer of an organic EL element, a method of sealing the organic EL element using a metal container or glass plate, or a method of placing desiccant within a space within which the organic EL element is sealed, have been used. However, in order to exploit such features of organic EL devices as light weight and thin shape, technology for sealing with a thin film, without using a desiccant, has attracted attention.
As sealing thin films, silicon nitride, silicon oxide-nitride, and other films have been used. However, in order to suppress damage to the light emission layer during fabrication of films of these materials, it has been necessary to hold increases in temperature at the fabrication surface to at least the glass transition temperature of the light emission layer or lower. Hence film fabrication methods developed for organic EL devices using semiconductor processes cannot be applied, and there has been the problem that a sealing thin film having adequate moisture-proof properties cannot be formed.
On the other hand, in Japanese Patent Application Laid-open No. 2005-285659 (Patent Document 1), a film is proposed as a sealing film which can be applied to organic EL devices, formed by a plasma CVD method, and having as main components silicon and silicon nitride. In Patent Document 1, it is disclosed that by means of a number of silicon atoms bonded with silicon atoms per number of silicon atoms bonded with nitrogen atoms of 0.6 or greater and 2.0 or less, as measured by X-ray photoelectron spectroscopy, the film exhibits excellent sealing properties.
In order to improve the aperture ratio of organic EL devices employing active matrix driving, in recent years devices with so-called top-emission type structures, in which light is extracted from the opposite side of the substrate on which TFTs and other switching circuits are fabricated, have become the mainstream. In this structure, a transparent electrode and sealing film are formed on the organic EL layer, and light emitted from the organic EL layer passes through the sealing film and is emitted to the outside. However, the sealing film in Patent Document 1 comprises a substantial quantity of silicon-silicon bonds, and so has low transmittance of visible light, and cannot be used as the sealing film in a top-emission structure.
An object of this invention is to provide an organic EL device having stability over the long term, by using a protective film having high transmittance for visible light and excellent moisture resistance. Another object of this invention is to provide a method for manufacturing an organic EL device such as that described above.